The present invention relates to composite electrolyte membranes for fuel cells and methods of making same. More specifically, the present invention is directed to proton-conducting membranes lot fuel cell applications. The present invention further describes materials which reach high intrinsic proton conductivity, and are suitable for use as electroiytic membranes in methanol fuel cells.
The need for pollution control stimulated the development of polymer electrolyte membrane fuel calls (PEMFC) and attracted an increasing interest particularly for the automotive and stationary power applications [1-3]. For example, Daimler Benz presented in 1998 a fuel cell powered car, NECAR II with a total electric power close to 50 kW. A fuel cell is an almost ideal energy source yielding a very high thermal efficiency and an essentially zero release of atmospheric pollutants. In transport applications, the direct methanol fuel cell (DMFC) is presently considered as the most appropriate and promising. Up to now, only perfluorinated ionomers (PFI) membranes were considered to meet the requirements of polymer electrolyte membrane (PEM) fuel cells, namely, a high proton conductivity, a high stability in the cell operating conditions and a high durability. Presently the commercial solid polymer electrolyte material used in PEMFC is either perfluorinated NAFION (Du Pont) or NAFION-like polymers [4] supplied by Dow, Asahi Glass (FLEMION) and Asahi Chemicals (ACIPEX). Unfortunately, these PEMFC limit large scale application due to a number of drawbacks. First of all, these ionomers are very expensive. For example, the manufacturer""s price for the NAFION membranes (Dupont de Nemours) scale exceeds 600 US$/m2. Other membranes of this kind (DOW membranes, RAI membranes, . . . ) are still more expensive (up to 2000 US$/m2). In fact, such membranes have been used for a long time in H2 fuel cells for application where cost was not a main criterion (e.g. spacecraft, submarines etc). In addition, a significant drawback of these materials is their high permeability to methanol which allows an easy transport of this fuel from the anode to the cathode. This phenomenon, also called methanol crossover, reduce significantly the cell performance and must be diminished if not eliminated before DMFC can be commercialized.
Currently the necessity to reduce the cost of PEM stimulates the development of new proton conducting polymers. New studies are also undertaken in order to rationalize the most efficient combination of properties of the perfluorinated ionomer (PFI) polymer, which make them efficient proton conductors, and develop new polymers with similar properties by a less expensive chemistry. As a result PFI NAFION membrane has been extensively studied and tested in low temperature fuel cell systems [5]. In this context, Ballard Advanced Materials"" group for the development of PEM membranes [6] recently developed a membrane based on a trifluorostyrene monomer called BAM3G (Ballard Advanced Material 3rd generation), which has demonstrated excellent performance and longevity of several thousand hours of operation.
The remarkable properties of PFI polymers lie in the combination of the high hydrophobicity of the fluorinated polymer backbone and high hydrophilicity of the sulfonic acid branches. The hydration of the PFI membrane is crucial for the performance of PEMFC since proton conductivity decreases drastically with dehydration. For instance, with NAFION membrane, which loses water above 80xc2x0 C., the conductivity drops to very low values above this temperature.
One more limitation associated with NAFION type PFI membranes [2,4] is the methanol crossover when used in the direct methanol fuel cell (DMFC). This results in a decreased fuel cell performance due to depolarization of the oxygen reducing cathode. A further drawback of the perfluorinated polymers is that they are not environmentally friendly, a criteria that will be important when fuel cells become mass-produced.
The above mentioned disadvantages of PFI membranes induced many efforts to synthesize PEM based on hydrocarbon-type polymers and brought about the emergence of partially fluorinated and fluorine free ionomer membranes as alternatives to NAFION membranes. Among them the membranes based on aromatic polyether ether ketone (PEEK) were shown to be of promise for fuel cell application [7-9], as they possess good mechanical properties, thermal stability, toughness and some conductivity, depending on sulphonation degree. Nevertheless, the proton conductivity of PEEK or SPEEK has yet to reach a level sufficient to enable an adequate performance in a fuel cell.
Efforts have thus been made to improve the proton conductivity to composite membranes. For example, the addition of solids such as zeolites or tin-mordenite was aimed at improving the performance of the composite membranes. Unfortunately, their presence in the membranes does not impart thereto a high enough proton conductivity to make them useful as a solid electrolyte in polymer electrolyte membrane fuel cells (Mikhailenko et al. 1997, Microporous Mat. 11:37-44).
xe2x80x9cPolymer Material for Electrolytic Membranes in Fuel Cellsxe2x80x9d by Yen et al., U.S. Pat. No. 5,795,496 is one such example of SPEEK with the aim of using it in fuel cells. The materials described in Yen et al. have low methanol permeability but high proton conductivity, and made from inexpensive, readily available materials. According to that invention, proton conducting membranes are formed based on a sulfonic acid-containing polymer. One preferred material is PEEK or PES. This invention is said to overcome disadvantages associated with the high cost of NAFION membranes and with its methanol permeability problems which allows for a substantial amount of fuel crossover across the membrane by using materials which were inexpensive starting materials and which enhanced protection against fuel crossover. In a particular embodiment, PEEK was sulfonated with H2SO4 to give H-SPEEK, a polymer which is soluble in an organic solvent and water mixture. While the inventors found that sulfonic acid increases the proton conducting performance of PEEK (the sulfonate groups are responsible for proton conductivity), it degrades the physical structure of the resulting membrane. Hence, the inventors developed a trade-off between the amount of sulfonation and appropriate physical structure by sulfonating less than one out of every three benzene rings. PBS was treated in an analogous manner. Yen et al. also teach methods of modifying the morphology of the processed polymers to limit the transport of methanol across the membrane (to reduce the free volume) by using zeolites tin motdeioite or the like. Unfortunately, such solids do not impart high enough proton conductivity to the composite membrane. However, Yen et al. do not teach a composite electrolyte membrane which reaches a high enough proton conductivity to be useful in PEM fuel cells.
During the last two decades solid electrolytes have attracted substantial attention owing to both their great potential in several electrochemical technologies, such as fuel cells, batteries and sensors, and the academic, interest in the phenomenon of fast ionic mobility in solids In spite of the fact, that a vast number of various proton conductors have already been identified, the development of chemically stable superionic conductors still remains one of the prime objectives among the current directions of research in solid state electrochemistry and materials science. Currently considerable efforts are being devoted to proton conducting salts of oxo acids including various hydrated and anhydrous phosphates
Heteropolyacids (HPAs) are known as the most conductive solids among the inorganic solid electrolytes. Nevertheless, the use of HPAs or other solid inorganic acids in polymers and their effect on membranes for fuel cells, for example, has not been significantly addressed to show that they could enable the production of an effective, high proton-conductive membrane for fuel cells. For example. heteropolyacid/polyethersulfone membranes are described in Park et al., 1996, Denki Kagaku 64:743-747. However, these membranes are more than likely impossible to use commercially due to their lack of stability under conditions of fuel cell use.
Boron phosphate (BPO4), a compound commonly synthesized from boric and phosphoric acids, has been widely used over the last three decades as an acidic catalyst in a number of reactions, including particularly dehydration reactions.
BPO4 belongs to the class of orthophosphates in which both P5xe2x88x92 and B3xe2x88x92 are tetrahedrally coordinated by oxygen.
Although the foregoing suggests that BPO4 in the presence of adsorbed water can possess interesting electrical properties, there were no attempts to look upon boron phosphate as being a solid proton conductor until the paper of Mikhailenko et al., 1998 (J. Chem. Soc., Faraday Trans. 94:1613-1618) which investigated the electrical properties of BPO4. It is taught therein that the conductivity of thermally treated boron phosphate is of only one order of magnitude inferior to that of such prominent solid electrolytes as hydrated heteropolyacids. At the same time, the chemically durability of BPO4 is far higher than that of HPAs, which are 100% soluble in water. Therefore, boron phosphate can be regarded as having some potential in electrochemical applications, such as fuel cells, hydrogen gas sensors and humidity sensors.
There thus remains a need to provide composite electrolyte membranes, which retain a proton conductivity which is high enough to make it useful in polymer material for electrolytic membrane in fuel cell PEMFC. There also remains a need to provide such composite membranes which provide a proton conductivity which is comparable to that of NAFION(trademark) membranes and in addition overcome disadvantages in the membranes of the prior art such as pollution, low thermal efficiency and substantial costs.
The present invention seeks to meet these and other needs.
The present description refers to a number of documents, the content of which is herein incorporated by reference in their entirety
The invention concerns a composite electrolyte membrane comprising a filler material which contributes to the enhancement of the protonic conductivity of the membrane. In a particular embodiment of the present invention the composite electrolyte membrane possesses a high enough proton conductivity to be an alternative energy source for stationary and mobile applications in a particularly preferred embodiment of the present invention the composite electrolyte membrane possesses a high enough proton conductivity to be used in methanol fuel cells.
The present invention further relates to composite electrolyte membranes comprising a polymer matrix and a filler material which contributes to the enhancement of the protonic conductivity of the membrane. In a particular embodiment the filler material contributes significantly more than the polymer matrix to the protonic conductivity of the membrane. In another embodiment, both the polymer matrix and filler material contribute significantly to protonic conductivity of the membrane. In a preferred embodiment of the present invention, the filler material is an inorganic solid acid. In a particularly preferred embodiment of the present invention, the polymer matrix of the composite electrolyte membrane is modified PEEK and the filler material is BPO4. In a more particularly preferred embodiment, the modified PEEK is sulfonated PEEK (SPECK).
The invention in addition relates to an environment-friendly composite electrolyte membrane comprising an environment-friendly polymer matrix and an environment-friendly filler material which contributes to the enhancement of the protonic conductivity of the membrane. In a preferred embodiment, the environment-friendly polymer matrix is a modified PEEK and the environment-friendly filler is an inorganic solid acid. In a particularly preferred embodiment of the present invention, the environment-friendly polymer matrix of the composite electrolyte membrane is SPEEK and the environment-friendly filler material is BPO4.
The invention further relates to composite electrolyte membranes for fuel cells, which remains efficient at a temperature above about 80xc2x0 C., preferably above about 90xc2x0 C., most preferably about 100xc2x0 C., and even more preferably between about 100xc2x0 C. and 120xc2x0 C.
In addition, the invention relates to fuel cell membranes comprising a solid electrolyte embedded in a polymer matrix, wherein the solid electrolyte contributes to the protonic conductivity of the membrane
The invention also relates to methods for preparing and pretreating an inorganic solid acid, so as to embed it in a polymer matrix for fuel cells.
In addition, the invention relates to methods for embedding a solid inorganic acid in a polymer matrix.
Before the present invention, solid fillers in composite membranes were not contributing sufficiently to the protonic conductivity of the membrane.
In accordance with the present invention, there is therefore provided a method of increasing the proton conductivity of a composite electrolyte membrane comprising an acidic polymer matrix, the method comprising an embedding of a proton conductivity effective amount of an organic solid acid in the matrix.
In accordance with another aspect of the present invention, there is provided a method of increasing a proton conductivity and/or a stability of a proton conductivity at a temperature above about 80xc2x0 C., in a polymer electrolyte membrane for fuel cell, comprising an embedding into the polymer of a proton conductivity effective amount of an inorganic solid acid.
While the composite electrolyte membranes of the instant invention have been demonstrated in particular with SPEEK in which BPO4 or heteropolyacids (HPAs) have been imbedded, the invention should not be so limited. Indeed, the present invention relates to composite electrolyte membranes in which a polymer matrix is embedded with a solid inorganic acid. Non-limiting examples of inorganic acids which could be used in accordance with the present invention include HPAs (also exemplified hereinbelow). Including tungtsophoric acid (Mikhailenko et al., 1997, Solid State Ionics 99 281-286), molybdophosphoric acid, and molybdosilicic acid; zirconium or titanium oxophosphates and sulphates; grafted silica materials such as for example ORMOSIL and ORMOCER ceramics, and mixtures of these inorganic acids. Broadly, the skilled artisan, to which the present invention pertains, will understand that any polymer matrix, and especially any sulfonated and/or phosphorylated polymer matrix having the requisite structural characteristic enabling the formation of a membrane having the requisite protonic conductivity, following the teachings of the present invention, can be used.
The skilled artisan to which the present invention pertains will understand however, that the inorganic solid acid should be chosen amongst the acids having a sufficient acidic strength to provide the necessary protonic conductivity to the polymer and membrane. The contribution of the inorganic solid acid to The total weight of the membrane should be below about 80% and preferably below 70%.
The skilled artisan should also comprehend that the present invention should not be limited to SPEEK. Indeed, other polymer matrices can be used in accordance with the present invention. Non-limiting examples thereof include polysulfones, polystyrenes, polyether imides, polyphenylenes, poly alpha olefins, polycarbonates and mixtures thereof. Similarly, the modification of these polymer matrices is not limited to sulfonation since, for example, phosphorylation could also be used.
In addition, it is herein demonstrated that an inorganic solid acid, such as BPO4 can be modified to enhance is protonic conductivity capacity and/or the stability thereof. The possibility to enhance the BPO4 inertness towards water was assessed by the introduction of aluminum as a stabilizing component. The resulting Al-BPO4 compound was used to monitor its conductivity by impedance spectroscopy as a function of Al loading, calcination temperature and water content. It was discovered that the partial replacement of boron with aluminum in boron phosphate brings about an increase in inertness of the solid towards water. In some cases it is achieved at the expense of a decrease in its conductivity. However, samples with Al/B (5/95 calcined at T less than 600xc2x0 C. where found to possess the same conductivity as pure BPO4, in spite of their lesser solubility compared to boron phosphate. This observation reinforces the view that a compromise between high proton conductivity and high stability of boron phosphate is feasible, and can be achieved, among other approaches, through chemical modification of the embedded solid.
In accordance with the present invention, it will be understood that further modifications can be made to the composite membrane. As will be seen below with PET, the incorporation of a basic compound into the acidic polymer matrix can improve its conductivity and/or stability. Non-limiting examples of such basic compounds which can be incorporated into the composite electrolyte membranes, in accordance with the present invention include polyethermide (PEI) and polyoxadiazole. PEI is a porous solid which can retain water and release it upon drying of the membrane, thereby maintaining a more efficient conductivity of the membrane with time (especially at higher temperatures, which tend to accelerate the drying and hence the diminishing of the conductivity of the membrane).
In addition, while solid fillers such as a zeolite material, (e.g. mordenite; see for example U.S. Pat. No. 5,795,496), or silicium material do not contribute to the protonic conductivity of the membrane, it should be clear to the skilled artisan, that the composite electrolyte membranes of the present invention could benefit from an addition of such fillers which can reduce methanol crossover (the filler serving as a sieve through which the methanol molecule cannot pass) and/or stabilize the membrane (e.g. by retaining water), thereby enhancing the preservation of the proton conductivity of the membranes. Numerous methods to incorporate fillers into the polymer matrix are known in the art (e.g. WO 90/29752, of Grot et al., published Sep. 26, 1996).
Always keeping in mind the need to maintain a structural and chemical stability for the resulting composite membrane, such fillers (basic fillers such as PEI or non-basic fillers) can be added to an amount of about 10% or less, and preferably to about 5% or less, based on the total weight of the membrane.
Properties which are preferred or required for direct methanol fuel cells (PMFC) arc as follows: a high proton conductivity of at least about 5xc3x9710xe2x88x922 S/cm in order to reduce Ohmic losses; a good mechanical resistance of films of 100 xcexcm thickness; a low permeation of reactants and products of the electrochemical combustion; a high chemical and electrochemical stability in the cell operating conditions; and a cost compatible with commercial requirements.
The terminology xe2x80x9cpossessing high enough proton conductivityxe2x80x9d, xe2x80x9cmaintaining a high enough proton conductivityxe2x80x9d and the like, should be understood by the person skilled in the art, to be dependent on the context of use of the composite electrolyte membranes in accordance with the present invention. For example, when using such membranes in a fuel cell, the conductivity of the composite membrane is preferably above about 2xc3x9710xe2x88x922 S/cm. More preferably, above about 5xc3x9710xe2x88x922 S/cm and even more preferably equal to or above 10xe2x88x921 S/cm. Since the conductivity of NAFION membranes often ranges between 2xc3x9710xe2x88x922 and 2xc3x9710xe2x88x922 S/cm, when referring to a conductivity which is comparable to that of NAFION membranes, the conductivity for the fuel cell composite membrane refers to a conductivity which is in that range or better.
It should be understood by the skilled artisan that since the structural and chemical stability of the membrane is critical to ensure an efficient and stable conductivity of the membrane, that the actual contribution of different materials in the composite membranes based on the total weight thereof needs to be taken into account.